NAME¶

pmemlog_create(), pmemlog_open(), pmemlog_close(), pmemlog_check() - create, open, close and validate persistent memory resident log file

SYNOPSIS¶

#include <libpmemlog.h>
PMEMlogpool *pmemlog_open(const char *path);
PMEMlogpool *pmemlog_create(const char *path, size_t poolsize, mode_t mode);
void pmemlog_close(PMEMlogpool *plp);
int pmemlog_check(const char *path);

DESCRIPTION¶

The pmemlog_create() function creates a log memory pool with the given total poolsize. Since the transactional nature of a log memory pool requires some space overhead in the memory pool, the resulting available log size is less than poolsize, and is made available to the caller via the pmemlog_nbyte(3) function. path specifies the name of the memory pool file to be created. mode specifies the permissions to use when creating the file as described by creat(2). The memory pool file is fully allocated to the size poolsize using posix_fallocate(3). The caller may choose to take responsibility for creating the memory pool file by creating it before calling pmemlog_create() and then specifying poolsize as zero. In this case pmemlog_create() will take the pool size from the size of the existing file and will verify that the file appears to be empty by searching for any non-zero data in the pool header at the beginning of the file. The net pool size of a pool file is equal to the file size. The minimum net pool size allowed by the library for a log pool is defined in <libpmemlog.h> as PMEMLOG_MIN_POOL.

Depending on the configuration of the system, the available non-volatile memory space may be divided into multiple memory devices. In such case, the maximum size of the pmemlog memory pool could be limited by the capacity of a single memory device. libpmemlog(7) allows building persistent memory resident logs spanning multiple memory devices by creation of persistent memory pools consisting of multiple files, where each part of such a pool set may be stored on a different memory device or pmem-aware filesystem.

Creation of all the parts of the pool set can be done with pmemlog_create(); however, the recommended method for creating pool sets is with the pmempool(1) utility.

When creating a pool set consisting of multiple files, the path argument passed to pmemlog_create() must point to the special set file that defines the pool layout and the location of all the parts of the pool set. The poolsize argument must be 0. The meaning of the mode argument does not change, except that the same mode is used for creation of all the parts of the pool set.

The set file is a plain text file, the structure of which is described in poolset(5).

The pmemlog_open() function opens an existing log memory pool. Similar to pmemlog_create(), path must identify either an existing log memory pool file, or the set file used to create a pool set. The application must have permission to open the file and memory map the file or pool set with read/write permissions.

Be aware that if the pool contains bad blocks inside, opening can be aborted by the SIGBUS signal, because currently the pool is not checked against bad blocks during opening. It can be turned on by setting the CHECK_BAD_BLOCKS compat feature. For details see description of this feature in pmempool-feature(1).

The pmemlog_close() function closes the memory pool indicated by plp and deletes the memory pool handle. The log memory pool itself lives on in the file that contains it and may be re-opened at a later time using pmemlog_open() as described above.

The pmemlog_check() function performs a consistency check of the file indicated by path. pmemlog_check() opens the given path read-only so it never makes any changes to the file. This function is not supported on Device DAX.

RETURN VALUE¶

On success, pmemlog_create() returns a PMEMlogpool* handle to the memory pool that is used with most of the functions from libpmemlog(7). If an error prevents any of the pool set files from being created, it returns NULL and sets errno appropriately.

On success, pmemlog_open() returns a PMEMlogpool* handle to the memory pool that is used with most of the functions from libpmemlog(7). If an error prevents the pool from being opened, or a pool set is being opened and the actual size of any file does not match the corresponding part size defined in the set file, pmemlog_open() returns NULL and sets errno appropriately.

The pmemlog_close() function returns no value.

The pmemlog_check() function returns 1 if the persistent memory resident log file is found to be consistent. Any inconsistencies will cause pmemlog_check() to return 0, in which case the use of the file with libpmemlog will result in undefined behavior. The debug version of libpmemlog will provide additional details on inconsistencies when PMEMLOG_LOG_LEVEL is at least 1, as described in the DEBUGGING AND ERROR HANDLING section in libpmemlog(7). pmemlog_check() will return -1 and set errno if it cannot perform the consistency check due to other errors.

CAVEATS¶

Not all file systems support posix_fallocate(3). pmemlog_create() will fail if the underlying file system does not support posix_fallocate(3).

SEE ALSO¶

pmempool(1), creat(2), posix_fallocate(3), pmemlog_nbyte(3), poolset(5), libpmemlog(7) and <https://pmem.io>